Diagnosis of an arterial sclerosis is carried out using an ultrasonic diagnostic apparatus that transmits an ultrasonic wave toward a body and obtains biological information based on an echo signal that has been received from the body. In making a diagnosis of arterial sclerosis, the intima-media complex thickness of a carotid artery is known as one of most important indices to the status of an initial atheroma sclerosis. The “intima-media complex” or “intima-media” is a generic term collectively referring to the intima and the media of a vascular wall that are present between the lumen and adventitia of a blood vessel. The intima-media complex thickness or intima-media thickness is generally called, and will also be referred to herein as, “IMT”.
FIG. 1 illustrates an intima-media 3 that is present between the lumen 1 and adventitia 2 of a blood vessel. In making an inspection, a lumen boundary 4, which is the boundary between the lumen 1 and intima of the blood vessel, and an adventitia boundary 5, which is the boundary between the media and the adventitia 2, are detected and have their thicknesses measured. The technique disclosed in Patent Document No. 1 is known as a method for measuring the IMT automatically.
FIG. 2 shows the intensity distribution of an echo for use to define the adventitia boundary and the lumen boundary according to the technique disclosed in Patent Document No. 1. As shown in FIG. 2, according to the method disclosed in Patent Document No. 1, two portions of the echo intensity distribution in the depth direction, of which the intensity variations are equal to or greater than a predetermined value, are regarded as potential candidates for boundaries, and the bigger one of the two is defined to be the adventitia boundary and the smaller one is defined to be the lumen boundary.